1. Field of the Invention
This invention relates generally to combat identification systems for the dismounted soldier and more particularly to a secure covert identification as friend or foe (IFF) system for interrogating a dismounted soldier with a coded infrared (IR) signal that is selectively retroreflected and encoded by the target when recognized as a valid challenge.
2. Description of the Related Art
Dismounted Armed Forces have an interest in the remote and secure identification of a person as friend or foe, during combat training exercises and in armed conflicts. Identification as friend or foe (IFF) systems are well-known in the art for military aircraft and other weapons systems. Such systems are useful for preventing action against friendly forces. The military platform commanders on a modern battlefield must accurately identify potential targets as friend-or-foe (IFF) when detected within range of available weapon systems. Such target IFF presents a difficult decision for a military platform commander, who must decide whether to engage a detected target while avoiding accidental fratricide. This problem is even more difficult for the dismounted soldier who may be moving covertly through an unknown combat zone at night with limited visibility. Simple visual assessments of other dismounted soldiers is not a reliable IFF method for military platforms or dismounted infantry.
The art is replete with proposals for IFF systems for military platforms in modern land battlefields. But commanders often still rely on low-resolution visual and infrared images to identify detected targets. Commanders often must operate under radio silence to avoid detection by an enemy. With infrared (IR) imagers alone, the identification of individual dismounted soldiers is not feasible, although the IR signatures of land vehicles may have some use. IFF systems that require one or more radio signals are limited in channel-capacity and must bear the overhead of selecting and/or awaiting an available battlefield channel before completing the IFF task. Active-response systems require the emission of a signal by the unknown respondent in response to a verified challenge, which may compromise the security of both interrogator and respondent. Active transponders are subject to capture and may be used for spoofing by the enemy in a battlefield or a combat training environment. Passive response systems rely on the return of an echo (reflection) of a challenge signal to the interrogator, but simple reflection schemes are easily compromised and more elaborate passive reflection schemes are still subject to intercept, compromise or capture for use by the enemy in spoofing the interrogator.
As described in U.S. Pat. No. 4,851,849 by Otto Albersdoerfer, a typical active IFF technique for vehicles is to equip a military vehicle with a transponder that emits a coded return signal when an interrogating radar pulse is detected by its receiver. As described in U.S. Pat. No. 5,686,722 by Dobois et al., a more sophisticated active IFF technique for vehicles uses a selective wavelength optical coding system with tunable optical beacons mounted on each vehicle. By spreading the optical broadcast energy into frequency in a precise manner, the beacon identifies the host vehicle to friendly receivers while remaining covert to the enemy.
As described in U.S. Pat. No. 4,694,297 by Alan Sewards, a typical passive IFF technique for vehicles is to equip a military vehicle with a passive antenna that reflects an interrogatory radar beam while adding a distinctive modulation by varying the antenna termination impedance responsive to evaluation of the interrogatory beam. A more sophisticated passive electro-optical IFF system for vehicles is described in U.S. Pat. No. 5,274,379 by R. Carbonneau et al. wherein each friendly vehicle is provided with a narrow-beam laser transmitter and a panoramic detector. If a vehicle detects a coded interrogator laser beam and identifies the code as friendly, it opens a blocked rotating retro-reflector to clear a reflection path back to the source, where it can be identified by another narrow field-of-view detector. A further modulation is also added to the reflected beam to identify the reflecting vehicle as friendly. If an improperly coded beam is detected, the transmission path is not cleared, thereby preventing reflection of that beam and warning is sent to the vehicle commander of an unfriendly laser transmission. Others have proposed similar passive optical IFF systems for vehicles, including Wooton et al. in U.S. Pat. No. 5,459,470 and Sun et al. in U.S. Pat. No. 5,819,164.
The art is less populated with IFF proposals for the lone dismounted soldier (the infantryman on foot). Whether in actual combat or in a training exercise, the dismounted soldier operates with severe weight limits and little onboard electrical power. The friendly foot soldier has no distinctive acoustic, thermal or radar cross-section that may be used to assist in distinguishing friendlies from enemies. But some practitioners have proposing IFF solutions for the dismounted soldier, both active and passive. For example, in U.S. Pat. No. 6,097,330, Kiser proposes an active IFF system for identifying concentrations of ground troops (or individuals) from an aircraft by interrogating a (heavy) human-mounted radio transmitter carried by one of the group with a narrow-cast optical signal. As another example, in U.S. Pat. No. 5,299,277, Rose proposes a compact active IFF system to be carried by each individual dismounted soldier for use in combat exercises or on the battlefield. The system includes a clip-on beacon and a hand-held (flashlight-style) or weapon-mounted detector. The beacon radiates a spread-spectrum low-probability-of-intercept (LPI) signal at optical frequencies that are selected to be invisible to the usual detectors present in the battlefield. Rose doesn't consider the problem of spoofing with captured devices. As yet another example, in U.S. Pat. No. 5,648,862, Owen proposes an active IFF system implemented by adding provisions for coded two-way transmissions to the night-vision systems often worn by dismounted soldiers. As a final example, in U.S. Pat. No. 5,966,226, Gerber proposes an active combat IFF system for each dismounted soldier that includes a weapon-mounted laser projector for interrogating suspected targets and a harness including means for receiving the interrogatory signal and means for responding with an encoded radio, acoustic or optical signal. But these proposals do not resolve the spoofing problem (through capture of a beacon or harness, for example). and are not particularly covert because the responding target generally broadcasts an active signal either continuously or in response to interrogation. Any IFF proposal employing broadcast signals also faces a battlefield channel capacity (or channel availability delay) problem as well.
There is still a clearly-felt need in the art for an IFF system for the dismounted soldier that provides true passive covertness and that cannot be spoofed under any battlefield conditions. The desired IFF system requires little power and is adapted to prevent any use of captured equipment or intercepted signal codes. Finally, the system should be inexpensive enough to permit equipping every soldier with the necessary interrogation and response equipment for combat exercises or actual battlefield conditions. These unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.